For power transistors, large Rdson cause larger power consumption. It is desirable to reduce the Rdson as much as possible to reduce device power consumption. The Rdson of a transistor usually can be simulated and calculated. For a 30V trench gate MOSFET with 0.8 micron pitch, the total Rdson in a square millimeter under a 10V is typically around 4.1 milliohm, while the Rdson of the silicon substrate is about 2 milliohm, which contributes almost 49% of the total Rdson; and the total Rdson in a square millimeter under a 4.5V is 5.7 milliohm, while the Rdson of the silicon substrate is 2 milliohm approximately, which contributes almost 35% of the total Rdson. Thus, reducing the silicon substrate can improve Rdson in the power device.
A plurality of etching methods of standard techniques can be used for etching the silicon substrate. Besides, electric-chemical etching method can also be used to etch the silicon substrate. The main principle of the electric-chemical etching method is to use a PN junction generated at the interface of an N-type semiconductor slab and a P-type semiconductor slab to implement electric-chemical etching on a semiconductor slab that needs to be etched under a reverse biased condition. As shown in the FIG. 1, the N-type semiconductor base plate and the P-type semiconductor base plate combined together are dipped into an etchant. The P-type semiconductor base plate, which needs to be etched, is exposed to the etchant. Then the N-type semiconductor base plate is communicated with an anode, while a cathode is arranged in the etchant. A reference electrode is also arranged in the etchant for reference. During the etching process, the etching stops when the wet etching arrives at the PN junction. The etching conditions can be monitored by measuring current ICE.
U.S. Pat. No. 6,111,280 discloses a gas sensor including at least one sensing area provided with a gas-sensitive layer, a MOSFET heater for heating the gas-sensitive layer to promote gas reaction with the gas-sensitive layer, and a sensor for providing an electrical output indicative of gas reaction with the gas-sensitive layer. The silicon substrate of the device in the sensing area is back-etched so as to form a thin membrane in the sensing area, which improves the gas detection sensitivity of the gas sensor.
U.S. Pat. No. 4,618,397 discloses a pressure sensor including diffused resistors formed in a front surface of a semiconductor crystal body, a rear surface of which is thereafter etched partly so as to put a part of the body into a membrane portion, which improves the pressure sensitivity of the pressure sensor.
U.S. Pat. No. 6,927,102 discloses a lateral power MOSFET device having an active region that includes a drift region with a portion of the semiconductor substrate below a portion of the drift region has been removed, which transversely reduces a parasitic capacitance of the power MOSFET. However, the resistances of the device are not reduced.